Data from: Strongly asymmetric hybridization barriers shape the origin of a new polyploid species and its hybrid ancestor

PREMISE OF THE STUDY: Hybridization between diploids and tetraploids can lead to new allopolyploid species, often via a triploid intermediate. Viable triploids are often produced asymmetrically, with greater success observed for “maternal-excess” crosses where the mother has a higher ploidy than the father. Here we investigated the evolutionary origins of Mimulus peregrinus, an allohexaploid recently derived from the triploid M. ×robertsii, to determine whether reproductive asymmetry has shaped the formation of this new species. METHODS: We used reciprocal crosses between the diploid (M. guttatus) and tetraploid (M. luteus) progenitors to determine the viability of triploid M. ×robertsii hybrids resulting from paternal- vs. maternal-excess crosses. To investigate whether experimental results predict patterns seen in the field, we performed parentage analyses comparing natural populations of M. peregrinus to its diploid, tetraploid, and triploid progenitors. Organellar sequences obtained from pre-existing genomic data, supplemented with additional genotyping was used to establish the maternal ancestry of multiple M. peregrinus and M. ×robertsii populations. KEY RESULTS: We found strong evidence for asymmetric origins of M. peregrinus, but opposite to the common pattern, with paternal-excess crosses significantly more successful than maternal-excess crosses. These results successfully predicted hybrid formation in nature: 111 of 114 M. ×robertsii individuals, and 27 of 27 M. peregrinus, had an M. guttatus maternal haplotype. CONCLUSION: This study, which includes the first Mimulus chloroplast genome assembly, demonstrates the utility of parentage analysis through genome skimming. We highlight the benefits of complementing genomic analyses with experimental approaches to understand asymmetry in allopolyploid speciation.研究背景：二倍体（diploid）与四倍体（tetraploid）间的杂交可通过三倍体（triploid）中间体产生新的异源多倍体（allopolyploid）物种。可存活三倍体的产生通常呈现不对称性，当母本倍性高于父本的“母本过量（maternal-excess）”杂交组合中，成功率更高。本研究针对近期由三倍体罗伯茨沟酸浆（M. ×robertsii）衍生而来的异源六倍体物种漂泊沟酸浆（Mimulus peregrinus）的进化起源展开探究，旨在明确生殖不对称性是否塑造了该新物种的形成过程。 研究方法：我们以二倍体亲本斑点沟酸浆（Mimulus guttatus）与四倍体亲本黄沟酸浆（Mimulus luteus）为材料开展正反交实验，以探究父本过量（paternal-excess）与母本过量杂交组合所产生的三倍体罗伯茨沟酸浆（M. ×robertsii）杂交种的存活能力。为验证实验结果能否预测野外实际的杂交模式，我们开展亲本分析，将天然分布的漂泊沟酸浆（M. peregrinus）种群与其二倍体、四倍体及三倍体祖先种群进行比对。我们从已有的基因组数据中获取细胞器序列（organellar sequence），并补充额外的基因分型数据，以此确定多个漂泊沟酸浆（M. peregrinus）与罗伯茨沟酸浆（M. ×robertsii）种群的母本起源。 关键结果：我们发现漂泊沟酸浆（M. peregrinus）的起源具有显著不对称性，但与常见模式相反，父本过量杂交组合的成功率显著高于母本过量组合。该实验结果成功预测了野外的杂交形成模式：114株罗伯茨沟酸浆（M. ×robertsii）个体中的111株，以及全部27株漂泊沟酸浆（M. peregrinus）个体，均携带斑点沟酸浆（M. guttatus）的母本单倍型。 结论：本研究完成了首例沟酸浆属（Mimulus）叶绿体基因组组装（chloroplast genome assembly），证明了通过基因组浅层测序（genome skimming）开展亲本分析的实用性。我们强调了将实验方法与基因组分析相结合的优势，以此阐明异源多倍体物种形成过程中的不对称性。